1. Field of the Invention
This invention relates to the composition of fluophosphate optical glass in which dispersion is particularly small, and the optical constant thereof is such that the refractive index nd=1.43-1.48 and the Abbe number .nu.d=85-97.
2. Description of the Prior Art
It is well known that the use of low dispersion glass in an optical system provides a small chromatic aberration which leads to obtainment of images of good quality. Particularly, fluophosphate optical glass exhibits relatively great abnormal partial dispersion and enables good correction of secondary spectrum and therefore, the effect thereof is further enhanced. As a glass containing a high amount of fluorine, there is known the so-called fluoride glass having beryllium fluoride BeF.sub.2 as the glass former. The optical constant thereof is such that nd=1.33-1.42 and .nu.d=90-105. However, beryllium has a strong toxicity and therefore, fluophosphate glass containing no beryllium and having phosphate added thereto as the glass former and having improved stability for devitrification is known. For example, in the P-Al-R.sup.II -F-O system (R.sup.II is an alkaline earth metal) as disclosed in Japanese Patent Publication No. 7430/1957, a glass composition of optical constant nd=1.45-1.55 and .nu.d=80-90 is known, but the Abbe number thereof could not equal that of fluoride glass having beryllium fluoride as the glass former. To overcome this demerit, materials having small contents of phosphor P and oxygen O and having great quantities of calcium compound and alkaline metal compound introduced thereinto have been reported. For example, Japanese Laid-open Patent Application No. 132014/1978 discloses a glass consisting of P.sub.2 O.sub.5 +As.sub.2 O.sub.3 6-12, R.sup.I.sub.2 O (R.sup.I is an alkaline metal) 3-15, CaO 22-26, Al.sub.2 O.sub.3 18-21, SrO+BaO 5-11 and F.sub.2 -O 27-30 (all being in percent by weight) and having an optical constant nd=1.40-1.44 and .nu.d=86-94. However, this glass has strongly toxic arsenic introduced thereinto as required and moreover, has a great deal of alkaline metal compound and therefore, the viscosity of its molten glass liquid has become very low and it has been very difficult to mold it during the cast.